An image captured by a camera device, such as a surveillance camera, may include a bright part (e.g. a sunny place) and a dark part (e.g. a shady place). In a case where a difference in brightness between the bright and dark parts is large, if the lens aperture value is set (adjusted) with reference to the bright part, the dark part becomes invisible. In contrast, if the lens aperture value is set (adjusted) with reference to the dark part, the bright part becomes invisible. Thus, conventionally, a camera device has been proposed that includes a function that performs exposure control using two image signals with different exposure times (a long exposure signal and a short exposure signal) and thereby expands the dynamic range (e.g. see Patent Literature 1).
In this conventional exposure control method, if the luminance level of a long exposure signal (also referred to as a LONG signal) exceeds a predetermined luminance level (saturation level), the long exposure signal and the short exposure signal (also referred to as a SHORT signal) are synthesized by a video processing, thereby expanding the dynamic range. More specifically, an image where the dark part is clearly visible (long exposure signal) and an image where the bright part is clearly visible (short exposure signal) are synthesized by a digital processing. Accordingly, an image where both the bright and dark parts are clearly visible can be acquired.
However, in the conventional exposure control method, in a case where the difference in brightness between the bright and dark parts is significantly large (e.g., a case where a bright light source (such as the sun or a light) is in a screen), a noise due to a short exposure signal (flickering) may be caused. In a case where the amount of light from a light source varies with a significantly short period (e.g., the amount of light of a fluorescent tube varies with a frequency of 60 Hz), if the exposure time of the short exposure signal is a certain length (e.g. 1/100 second), the amount of light per frame is flattened (averaged) to a certain extent. More specifically, although the amount of light from the light source varies with a period of 1/60 second, the amount of light is integrated for an exposure time of 1/100 second. Accordingly, the integrated value of the amount of light becomes a substantially same value among frames. However, in the case where the difference in brightness between the bright and dark parts is significantly large, the exposure time of the short exposure signal becomes significantly short (e.g. 1/1000 second). In this case, the variation of the amount of light becomes perceptible. That is, even if the amount of light is integrated for the significantly short exposure time of 1/1000 second, the integrated value of the amount of light also varies for every frame, which causes a noise on the screen (flickering).
Incidentally, in a case of employing a camera device as a surveillance camera, the “face of a figure” taken in a captured image (surveillance image) is a very important part. It is necessary to suppress a noise in this face part as much as possible. On the other hand, even if illumination such as of a light appears in the surveillance image, the part of the illumination is not so important. However, in the conventional exposure control method, in a case where a bright light source (e.g. a light) appears together in a surveillance image, the dark part (the face part which is very important) includes the aforementioned noise (noise due to the short exposure signal) in order to make the bright part (the part of the light source which is not important) clearly visible.